Object-Oriented Programming

Java is built on four OOP pillars — encapsulation, inheritance, polymorphism, and abstraction. Understanding these well is not just required for interviews; it directly determines whether the code you write is maintainable, testable, and extensible. Modern Java (16+) adds records and sealed classes that reshape how you model data.

Note: Clarifications — this domain includes JDK-specific features (records, sealed classes). Each topic indicates required JDK version when relevant; check the linked JEPs and language docs before using a feature in production.

What You'll Find Here

Topic	Description
Classes & Objects	Blueprints vs. instances — fields, methods, constructors, this.
Encapsulation	Access modifiers, getters/setters, immutable classes.
Inheritance	extends, method overriding, super, Liskov Substitution Principle.
Polymorphism	Compile-time (overloading) vs. runtime (overriding) dispatch.
Abstraction	Abstract classes vs. interfaces; when to use each.
Records (Java 16+)	Concise immutable data carriers; compact constructors.
Sealed Classes (Java 17+)	Restricted hierarchies enabling exhaustive switch.

Learning Path

1. Classes & Objects — understand the class/instance relationship and object construction.
2. Encapsulation — learn to hide state and expose intent through controlled access.
3. Inheritance — study when to extend vs. compose; the classic mistake is over-inheriting.
4. Polymorphism — this is the concept most interviews probe with tricky overloading/overriding questions.
5. Abstraction — distinguish abstract classes from interfaces; learn when default methods change the decision.
6. Records (Java 16+) and Sealed Classes (Java 17+) — Java 17+ additions every modern Java developer needs.

Related Domains

• Core Java — language primitives and control flow that OOP builds on.
• Java Type System — generics and type bounds are used throughout OOP hierarchies.
• Java Design Patterns — GoF patterns are OOP in action.